Plastic parisons or preforms are conventionally reheated and formed into hollow plastic articles by stretch blow molding in a blow mold using a stretch rod and high pressure fluid.
The preforms are preheated in ovens immediately prior to being transferred into a blow mold. Incorporated with the blow mold are stretch rods which typically enter the preform from its open end and extend during the blow stage to cause biaxial orientation of the blown article. The motion of the stretch rod should be synchronized with the blowing of the high pressure fluid so that the preform is stretched along its longitudinal axis by the rod and simultaneously or sequentially stretched perpendicular to this axis by the pressure of the incoming fluid to orient its molecules for greater strength.
A two stage reheat blowing machine of the type described in U.S. Pat. Nos. 4,793,960, 4,690,633 and 4,522,581 uses a series of pallets to carry preforms through a series of processing stages: thermal conditioning, stretch-blow molding and strip-off or ejection. At the stretch-blow molding station it is important that the preform is accurately aligned with the blow mold to insure that the finished blown article is symmetrically formed with respect to the neck finish. It is also important that the means used to seal the interior of the preform effectively prevent the escape of the high pressure, typically 700 psi, fluid used to blow the preform within the closed mold. Loss of the fluid reduces the effectiveness of the blowing, creates noise, and may cause the neck finish to be deformed if leakage occurs in that area.
Prior art machines for stretch blowing and using preforms mounted on traveling pallets by means of mandrels include U.S. Pat. Nos. 4,505,664, 4,426,202, 4,470,797, 4,487,568 and 4,456,447. Usually the mandrel is fitted inside the preform's neck finish with a minimum gap to reduce air leakage. The '568 and '447 patents show external latches to hold the preform on the mandrel and the '797 patent shows a spring loaded detent for the same purpose. In these cases the preform is depending from the mandrel/holder and unless restrained would possibly fall off, given the amount of machine vibration and motion that it is subjected to.
Prior art machines simply close the blow mold halves trapping the preform's neck finish between the mold halves and the inner mandrel, thus initially achieving an air tight seal. However, the high pressure blowing fluid may cause the neck finish to separate from the mandrel and allow the fluid to leak, and also act on the inner wall of the neck finish causing it to deform. This deficiency is particularly prone to occur when large diameter preforms are being blown. Here the large projected area of the neck finish can be easily deformed when exposed to the blowing fluid. Prior art machines rely on the engagement of the mold directly with the preform to align the preform within the closed mold. In some cases the preform may not be perfectly aligned, or may be moved off center by the action of the stretch rod acting inside the preform during stretching. Consequently the blown article's body may not be aligned with its neck finish. The '664 reference forms an internal thread in the blown part and the blow nozzle must be unscrewed from the part before the mold can be opened. U.S. Pat. Nos. 4,818,212 and 4,036,573 also show typical stretch blow nozzle arrangements.
Accordingly, it is a principal object of the present invention to provide an apparatus for forming hollow plastic articles including a blow mold which aligns and clamps the parison to the blow mold in a convenient and effective manner.
It is a further object of the present invention to provide an apparatus as aforesaid adaptable to various size preforms and which permits the use of high pressure fluid without leakage.
Further objects and advantages of the present invention will appear hereinbelow.